1. Field of the Invention
The present invention relates generally to a dielectric optical waveguide device, and more particularly to an improved technique for providing a high reliability and long service life of such a dielectric optical waveguide device.
2. Description of the Related Art
An optical waveguide device is known as a kind of optical device such as optical modulator. The optical waveguide device has an advantage such that it can be easily reduced in size and weight from a viewpoint of structure and can be mass-produced by using a planar technique or the like. In particular, a ferroelectric optical waveguide device, which is constructed of a substrate formed of ferroelectric crystal, an optical waveguide formed in the substrate by thermal diffusion of metal or oxide thereof, and a pair of metal electrodes arranged on the substrate, can control a refractive index according to an applied voltage, so that the ferroelectric optical waveguide device is applied to various optical devices.
A conventional dielectric optical waveguide device is constructed, for example, of a substrate formed of ferroelectric crystal, an optical waveguide formed in a part of the surface of the substrate by thermal diffusion of metal or oxide thereof, an insulating buffer layer formed on the surface of the substrate in which the optical waveguide is formed, a semiconducting film formed on the buffer layer, and a pair of metal electrodes arranged in spaced relationship from each other on the semiconducting film.
The reason why the insulating buffer layer is formed on the substrate is to reduce light absorption to the electrodes, and the reason why the electrodes are located on the semiconducting film is to avoid electric charges generated by a pyroelectric effect due to a temperature change that will be concentrated under the electrodes. The semiconducting film is formed of silicon (Si) in view of manufacturability, stability, etc., and the electrodes are formed of gold (Au).
In the conventional dielectric optical waveguide device, however, a solid-phase alloying reaction occurs at a joined portion of the semiconducting film of silicon or the like to the electrodes of gold or the like. In the case where the solid-phase alloying reaction between silicon and gold occurs, gold silicide is formed. This reaction proceeds with the passage of time to cause a problem that the pair of electrodes are finally short-circuited. In particular, the higher the temperature, the faster the progress of the solid-phase alloying reaction, so that the above problem becomes more serious.
In general, the solid-phase alloying reaction is a chemical change occurring between a metal solid and another solid joined thereto, that is, a reaction wherein metal molecules or atoms of the metal solid are diffused into the solid joined thereto to thereby cause the progress of alloying. In general, the higher the temperature, the higher the rate of the progress.